Redox-Based Separation of Americium from Lanthanides in Sulfate Media

Abstract

In the development of advanced nuclear fuel cycles, a primary motivation is the reduction of the long-term radiotoxicity of the wastes. From a few decades to several thousand years after removal of the fuel from the reactor, americium (Am) dominates the radiotoxicity of used fuel, thus its transmutation represents an attractive option for improved management of the residues. However, every viable scenario for transmutation of Am demands some degree of separation of Am from fission product lanthanides. Partitioning of Am from curium further simplifies the transmutation process. The mutual separation of these elements is very challenging due to their similar chemistry. The focus of this work is on the utilization of the upper oxidation states of americium (Am(V/VI)) to facilitate a more efficient separation of Am from fission product lanthanides and curium. A minimum 90% efficient separation of americium from the lanthanides and curium has been achieved in the laboratory by selectively oxidizing trivalent Am to the hexavalent state using Na₂S₂O₈. At equilibrium, lanthanides are precipitated as sodium lanthanide sulfate double salts, while oxidized Am species remain predominantly in the supernatant phase. Trivalent curium is readily coprecipitated with the NaLn(SO₄)₂ solid. Parallel studies of oxidized uranium, neptunium, and plutonium solutions support the conclusion that Am(VI) has sufficient stability to allow the separation to be completed in a reasonable time frame. A particular advantage of this approach is the absence of readily oxidized species that can quickly reverse the oxidation of Am and thus negate the separation. A variety of chemical and physical characterization techniques have been applied to profile the performance of the system.

Keywords:

• americium
• lanthanides
• persulfate
• solid-liquid separation

ACKNOWLEDGEMENTS

This research was conducted at Washington State University and at the Idaho National Laboratory with funding provided by the U.S. Department of Energy, Division of Nuclear Energy Science and Technology, University Nuclear Energy Research Initiative under project number DE-FC07-05ID14644.

Notes

¹All values μ = 1 and t = 25°C except NaEu(SO₄)₂ (s).

²NaEu(SO₄)₂(s) – value extrapolated from literature data (22,23) – μ = unknown and t = 20°C.

³Stability constants for are found in Chemical Thermodynamics of Uranium (24).

⁴Stability constants for Eu³⁺ are found in the Martell and Smith Database (25).

⁵Eu hydrolysis products .

⁶ hydrolysis products .

⁷Eu sulfate complexes .

⁸ sulfate complexes .

⁹Solubility product NaEu(SO₄)_2(s): .